Compression ignition engines provide advantages in fuel economy, but produce both NOx and particulates during normal operation. New and existing regulations continually challenge manufacturers to achieve good fuel economy and reduce the particulates and NOx emissions. Lean-burn engines achieve the fuel economy objective, but the high concentrations of oxygen in the exhaust of these engines yields significantly high concentrations of NOx as well. Accordingly, the use of NOx reducing exhaust treatment schemes is being employed in a growing number of systems.
One such system is the direct addition of a reducing agent or reductant, such as ammonia gas to the exhaust stream. It is an advantage to deliver ammonia directly into the exhaust stream in the form of a gas, both for simplicity of the flow control system and for efficient mixing of the reducing agent, ammonia, with the exhaust gases. The direct use of ammonia also eliminates potential difficulties related to blocking of the dosing system, which may be caused by precipitation or impurities, e.g., in a liquid-based urea solution. In addition, an aqueous urea solution cannot be dosed at a low engine load since the temperature of the exhaust line would be too low for complete conversion of urea to ammonia (and CO2).
A couple specific challenges with the direct injection of ammonia relate to dispersion and mixing of the reducing agent with the hot exhaust gases. The dispersion issue considers how to deliver or spread ammonia to the greatest volume of flowing exhaust, while the mixing issue questions how to create the most homogenous mixture of exhaust and ammonia to facilitate NOx reduction.
Thus, the present system provides both a device for adequately dispersing and sufficiently mixing a reductant, such as ammonia, into an exhaust gas stream of a vehicle. These and other problems are addressed and resolved by the disclosed system and method of the present application.